The overall objective of this research is to establish a scientific understanding of the biomechanical factors that govern walking performance in the elderly. Walking speed is a common clinical measure of gait performance. Slow walking speed has been associated with falls, can contribute to physical disability and is one of the primary criteria used to describe the syndrome of frailty. Two factors that may contribute to slow walking are reduced muscle strength and decreased flexibility. Intervention studies have shown that elderly adults can often achieve remarkable gains in strength and flexibility through exercise. However, such gains have often not translated into faster walking speeds. These inconsistent outcomes may stem, in part, from a limited understanding of the specific factors that constrain elderly gait performance. This study will use forward dynamic simulations of subject-specific walking patterns to describe the causal relationship between muscle excitations and movement. Analysis of simulations of elderly adult gait will be used to rigorously investigate biomechanical factors that may limit gait speed. Aim 1 will examine the contributions of passive hip flexor stiffness to the multi-joint movements seen in gait. The hypothesis to be tested is that compared to young adults, elderly adults will exhibit a greater reliance on hip flexor tightness to assist swing leg movement as gait speed is increased. Aim 2 will determine whether gait speed is limited by ankle plantarflexor power capacity. The hypothesis is that healthy elderly adults will not demonstrate limitations in ankle power during walking. However as gait speed is increased, it is hypothesized that elderly adults will exhibit an increased dependence on hip muscles to compensate for functional limitations of the ankle plantarflexors to generate forward acceleration. Completion of these aims is an important step toward characterizing the limiting factors to gait among impaired elderly and developing targeted interventions for improving gait performance.